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Bonding behaviour and surface adhesion mechanisms of tetraethyl orthosilicate silicon oxide films are investigated. Prior to the bonding, infrared absorption spectroscopy was used to assess chemical composition of the bonding layers. The incorporation of -OH groups during the deposition process and the moisture absorption is shown and a specific effect of the applied RF power is highlighted. A strong correlation is found between trapped species and the evolution of the bonded layers during subsequent thermal annealing. The first observed phenomenon is an overall hardness reduction of the film deposited at low RF power which results in an increase of local adhesion area, hence an enhancement of the bonding energy. In the meantime, in this configuration water production is promoted in the volume of the film through silanol condensation and silicon oxidation occurs at the interface between the bonding layer and the silicon bulk. As a by-product of this reaction, hydrogen is released and it migrates towards the bonding interface. As a consequence, defects appear at the bonding interface. Thanks to the use of a stop barrier at the bulk interface, silicon oxidation is prevented, defect free bonding is obtained and the described scenario is confirmed.